Purpose of sampling

How important is the sample design to the validity of your data? Explain.

Provide an example where a sample could misrepresent the validity of the data.

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What is the purpose of sampling?
A sample is a finite part of a statistical population whose properties are studied to gain information about the whole(Webster, 1985). Hence sample represents population and is a subset of population.
As per socialresearch, " Sampling is the act, process, or technique of selecting a suitable sample, or a representative part of a population for the purpose of determining parameters or characteristics of the whole population."
Purpose of sampling is as follows:
As per web.utk.edu, "The purpose of sampling is to make generalizations about the whole [the population] which are valid [accurate] and which allow prediction. If this spoonful needs salt, then it's likely that this would be true for others as well."

a) Sampling is used to study the whole population in less time and in an inexpensive manner.
b) Sampling helps in utilization of resources in optimal manner.
c) Sampling is also used when it is physically impossible to conduct a census because of huge size of population. For example if one has to understand the buying behavior of Indian middle class consumer. The population of Indian middle class consumer is huge as its above 700 million and hence in these cases sample will be the only solution.
d) Sampling is also useful in the ...